Plaster is still used to tape the seams between the pieces of drywall despite the invention of drywall and other types of wallboard which was meant as an alternative. To get those separate pieces to look like one smooth wall, you must connect them with several increasingly thin and wide seams of joint compound (a form of plaster), sanding each layer smooth. The process is a messy one, especially during the sanding, so seal off the rest of your house from the room where you’re working.
Spread joint compound over the seams with drywall knife, making a path of compound about six inches wide. Press and spread the compound enough that it just conceals the mesh tape and is reasonably smooth on top. Spread a golf-ball-sized dab of compound over each screw hole in the wallboard as well. Let dry for a day.
Sand the seams with your drywall sander, just enough to smooth out imperfections. Also sand smooth the compound over the screw holes. Spread another coat of compound over the seams---a little thinner but covering a wider area than the first---about eight to 10 inches wide. Let dry.
Repeat the process a third time, sanding the seams and then spreading the widest and thinnest coat of compound. Make the final seam about 12 inches wide and as smooth as you can get it. Let it dry.
Sand the third coat, getting the walls as smooth as you can. Wipe off the dust with a dry cloth. The wall is now ready for priming and painting.